The present invention relates to data communications equipment, e.g., modems, and, more particularly, to the equalization of signals in a data communications system.
Conventionally, a receiver employs an adaptive decision feedback equalizer (DFE) to compensate for distortion introduced by the communications channel. However, the use of a DFE introduces "error propagation" effects in the receiver. As such, it is known in the art to use pre-equalization with modulo arithmetic (e.g., Tomlinson filtering) in the far-end transmitter in order to mitigate, if not eliminate, the problem of error propagation in the receiver. This pre-equalizer uses equalizer coefficient values communicated from the receiver, typically over a reverse channel. These coefficient values are generated in the receiver as the result of an initialization phase, or training, between the far-end transmitter and the receiver.
However, if the response, i.e., characteristics, of the communications channel changes significantly, the pre-equalizer will not be able to compensate for the error propagation problem in the receiver. As a result, a re-train is required so that the receiver can generate a new set of pre-equalizer coefficients, which must be then sent back to the far-end transmitter. Unfortunately, each re-train takes time to both calculate the pre-equalizer coefficients anew and to communicate them back to the far-end transmitter over what is typically a low bandwidth reverse channel.